Bathroom tissue must reconcile several conflicting properties: bath tissue must be strong, soft, flushable, dispersible and degradable. Achieving desirable combinations of these properties at an economically viable cost is a considerable challenge.
However, adding resistance to wet abrasion as an additional and conflicting property to those previously mentioned, poses an even tougher technical challenge. Construction of a tissue which has sufficient wet strength so that it can be used premoistened, inherently conflicts not only with flushability and dispersibility, but also with retaining sufficient softness to be used either premoistened or dry.
In order to provide a household bathroom tissue which is acceptable to consumers, it is necessary to provide a soft tissue which has sufficient dry tensile strength for normal use. In addition, it is necessary that the tissue is sufficiently dispersible for flushing, in reasonable quantities, in typical household toilets, while providing a tissue with sufficient degradability to be accommodated in septic systems. Conventional bathroom tissue does not possess sufficient resistance to wet abrasion to be suitable for use premoistened without tending to pill or shred.
Usually, cleansing of the perineum and adjacent regions of the human body is performed with bathroom tissue in a dry condition. Dry tissue does not always cleanse these regions as thoroughly as may be desired. Some users would prefer to use a bidet to assist with the cleansing of these regions for a feeling of extra cleanliness. However, if an individual uses conventional bathroom tissue after the perineum and adjacent regions are thoroughly wet or proceeds to moisten the tissue prior to use of the tissue, known bath tissues, even those few brands having significant wet strength to retain some reasonable structure, have a tendency to pill.
Pilling is a phenomenon occurring during use wherein small balls of tissue cling either to the surface of the tissue or to the user, possibly leading the tissue to shred before cleaning is complete. Such a condition is not desirable to most users. One purpose of this invention is to provide a flushable, sewer and septic-compatible tissue product which may be moistened before use and still retain sufficient softness, strength and resistance to pilling to be used in cleaning.
One manner of adding wet strength to a product is to add "permanent" wet strength. Permanent wet tensile strength would normally interfere with both the dispersibility and degradability of the product and thus prevent the tissue from being compatible with a septic system. In addition, permanent wet tensile strength can often interfere with the flushing of the tissue in a typical household toilet, either by clogging the bowl or by being retained within the pipeline connecting the house to the sewer, thus causing clogging, particularly, as is often the case in older homes, when tree roots are present.
Conventionally, wet tensile strength is obtained in a paper product by adding, to the paper furnish, a permanent wet strength resin or agent, such as the polyamide epichlorohydrin resins sold by Hercules under the trademark KYMENE.RTM.. At least two mechanisms by which wet strength resins act have been postulated. One holds that wet strength resins form covalent bonds between adjacent fibers, while another holds that wet strength resins form a water resistant network over the hydrogen bonds formed between adjacent paper fibers, thus preventing water from breaking the hydrogen bonds. In a permanent wet strength product, the strengthening effect does not decay with time. Accordingly, paper products produced with permanent wet strength resins would not normally be acceptable for use in a conventional household toilet or for use with a septic system.
An alternative to providing permanent wet strength is to provide a temporary wet strength. To provide temporary wet strength, specialized temporary wet strength resins are incorporated into a cellulosic web. The nature of the resin chosen does not seem to be critical provided it contains aldehyde moieties and provides wet strength properties as described herein. Suitable products are usually water soluble aldehyde moiety containing polyols, monomers, cyclic ureas and mixtures of these. Typically, these chemical moieties are dialdehydes or water soluble organic polyols comprising aldehydic units. Although wishing not to be bound by any theory, it is thought that these polymers or aliphatic dialdehydes form hemiacetal linkages with the cellulose and that these hemiacetal linkages hydrolyze at a moderate rate when immersed in water, so tissues incorporating these resins have considerable initial wet strength, but after only a few minutes, the wet strength drops to some suitably low value to make the tissue flushable.
In practice, the initial wet strength of tissues made using these wet strength agents tends to increase moderately over the first several days subsequent to manufacture thereof. In our experience, wet strength tends to be fairly well leveled out within about a week after manufacture, so throughout this specification and claims, where we refer to wet strength, that wet strength should be understood to be wet strength as obtained after about a week of aging unless the context clearly indicates otherwise.
U.S. Pat. Nos. 3,096,228 and 2,622,960 disclose the use of glyoxal to improve the wet strength of paper products. The conditions under which glyoxal is applied to the web in these relatively old references tend to produce products which do not meet the five properties set forth for the tissue of this invention.
In U.S. Pat. No. 2,622,960 to Woods et al., paper is obtained by saturating a preformed and dryed sheet by immersion or spraying with an aqueous solution of glyoxal and subsequently heating the treated sheet at a temperature of at least 212.degree. F. This process has disadvantages when employed in the manufacture of toilet tissue, facial tissue, and light weight single ply towels since it tends to embrittle these light weight paper products causing a loss in tear strength of the web. These disadvantages are discussed in the prior art reference, Day et al. U.S. Pat. No. 3,096,228.
In order to address the shortcomings of Woods et al., Day et al. discloses a process for adding glyoxal to a dry absorbent paper web, having a moisture content of about 3 to 7% by weight based on the weight of bone dry paper, so that the final moisture content of the web is more than 4% and not more than 20% by weight. By storing the paper at this moisture content at room temperature, wet tensile strength is developed in the web by migration of glyoxal throughout the web. Consequently, paper rolls must be stored at least one day before converting in order to develop sufficient product wet tensile strength, or paper rolls must be converted into product form under mill condition such that initial web moisture content is maintained in the converted product package for at least 24 hours. In either case, logistical and/or environmental problems arise in the paper mill. Furthermore, the high moisture levels usually greater than or equal to 8-10% required in U.S. Pat. No. 3,096,228 to Day et al. tends to relax the stretch in a creped web (i.e. cause stretch pullout) and weaken the web, making converting on modern continuous winders difficult or impractical.
The present invention clearly distinguishes over these prior art references by the application of uncharged chemical wet strength agents before or after the Yankee pressing roll (16) to a wet fibrous web and thereafter drying and creping said web. This process leads to an unexpected enhanced temporary wet strength absorbent product without the negative aspect of requiring chemical migration by storage at high humidity levels. Without being bound by theory, we believe the addition of uncharged chemical wet strength agents to a web before and/or after a papermachine Yankee pressure roll allows for chemical migration within the sheet--ultimately enhancing wet tensile strength.
The hydraulic spray units utilized in U.S. Pat. No. 3,096,228 when applied to a dry sheet according to the procedure disclosed in that prior patent, will produce nonuniform paper products, particularly when glyoxal is sprayed before embossing. This procedure tends to lead to glyoxal build up on the finished rolls creating additional processing problems.
While at least one brand of commercially available bath tissue possesses some degree of temporary wet strength, it appears that the manufacturer's purpose in including temporary wet strength in those products may be to counter the effects of the wetting which occurs during normal use. Merely adding a temporary wet strength agent to this tissue does not render it suitable for use in a premoistened condition. When attempts are made to use this tissue after premoistening, the tissue "shreds" and "pills" quite severely. Thus, rather than providing enhanced cleaning, attempted use of these products in a premoistened condition often leaves considerable detritus of shreds and pills of paper on the area that was to be cleaned. When the area to be cleaned is covered in this detritus of shreds and pills, the purpose of premoistening the tissue is largely lost.
Unlike prior art tissues, the present invention provides a tissue which (i) has sufficient wet strength and resistance to wet abrasion so that it can be used premoistened; (ii) is flushable; (iii) is dispersible and biodegradable; (iv) has dry strength comparable to premium bath tissue; and (v) has softness comparable to modern premium bath tissue.
The tissue of the present invention reconciles these conflicting objectives by providing a tissue having a glabrous surface coupled with an initial normalized temporary wet strength of at least about 75 g/3 inches, preferably about 105 grams/3 inches as measured using the Finch Cup method for an 18.5 lb/3000 sq ft ream. The tissue of the present invention further exhibits a wet-to-dry CD (Cross Direction) tensile strength ratio of at least about 18%, preferably over 20%. Temporary wet strength is provided by use of a temporary wet strength chemical moiety added to the web, before the pressing roll (16) on the air side of the sheet, after the pressing roll (16) or on the Yankee (26) surface. This moiety generally has no charge and therefore is applied after the web has been formed. The chargeless chemical moiety includes aldehydes, aldehyde containing polyols, polymers, cyclic ureas and mixtures of these and can be used in combination with cationic starches, and optionally, a cationic softener/debonder to create a prewettable high softness tissue or towel having the desired physical parameters. A softener/debonder can be used directly with the chargeless aldehydes, and chargeless aldehyde containing polyols, polymers, cyclic ureas, and mixtures of these or they can be used in combination with the cationic starches. In this invention the primary wet strength agents are the uncharged aldehydes, and the uncharged aldehyde containing polyols, polymers and cyclic ureas or mixtures of these. The starches and softeners/debonders are utilized to obtain specific properties for certain specialized applications.
In our process the wet strength and dry strength can be controlled independently by balancing the amount of chargeless chemical moieties added to the web with the cationic strength enhancing agents added to the furnish. To further fine tune our system, we optionally utilize cationic softeners/debonders. These can be added to the furnish after the starch has been mixed with the furnish or sprayed on the web before or after the pressing roll. In our process cationic softeners/debonders need not be used if cationic strength enhancing agents such as starch have not been added to the furnish. In some instances, we use the chargeless chemical moieties in combination with cationic softeners/debonders, this combination functions as a temporary wet strength agent.
Simply adding a quantity of temporary wet strength resins to conventional furnishes for tissue does not guarantee that the product will be well suited for use premoistened. The present inventors have found that when the tissue has both a glabrous surface and a normalized CD wet tensile of at least about 75 g/3 inches, preferably 105 g/3 inches, as measured by the Finch Cup Test ("FCT") at a basis weight of about 18-19 lbs/3000 sq ft ream, the tissue will not typically pill or shred when an attempt is made to use it premoistened.
We have found that once the absolute (not-normalized) CD wet tensile of each sheet drops to about 36 g/3 inches or less, the sheet does not usually have sufficient integrity to survive normal use when wet even though the sheet may not pill if handled gingerly enough to avoid tearing the sheet. Throughout this application, where a normalized wet tensile strength is mentioned, it should be understood that the tensile strength is as determined using the Finch Cup procedure in which a 3 inch sample of converted ready-to-use product having a basis weight of 18.5 lb/3000 sq ft ream, (single ply or multi-ply as the case may be) is clamped in a special fixture termed a Finch Cup. The sample is then immersed in standard tap water and tensile tested at the indicated time after immersion. For initial wet tensile strength, the measurement is conducted 5 seconds after immersing in water. We prefer use of this procedure as we have found that the results obtained using the FCT are reasonably reproducible.
Since the critical factor with regard to pill formation seems to be the degree and strength of the internal bonds between the fibers in the sheet, for basis weights other than 18.5 lb/3000 sq. ft. ream, the critical cross direction (CD) tensile strength values (75 g/3 inches or 105 g/3 inches and so forth, as the case may be) should be adjusted proportionally to the basis weight i.e., normalized. For example, a 9.25 lb/3000 sq. ft. ream sheet having a CD wet tensile of about 52.5 g/3 inches will perform satisfactorily as the CD wet tensile is proportionally the same as an 18.5 lb/3000 sq. ft. ream sheet having a CD wet tensile of 105 g/3 inches and, accordingly, the normalized CD wet tensile of this 9.25 lbs/3000 sq ft ream would be 105 9/3 inches. This conforms well with our experience in which single plies of 9.25 lbs/3000 sq. ft. ream tissue have been satisfactory at CD wet tensile strengths of 66 and 44 g/3 inches, while single plies having a CD wet tensile of 36 g/3 inches fail by shearing without leaving pills.
The set strength values provided herein have been selected based upon standard tap water, however, it should be understood that water quality may affect the initial cross direction (CD) tensile wet strength values, as well as the decay rates. Furthermore, in an aqueous medium having been adjusted for pH or in a nonaqueous medium, the values and decay rates may shift. Such shifts are contemplated herein and are within the scope and spirit of the present invention.
To ensure that the tissue product will be sufficiently flushable to avoid requiring an excessive number of flushes to clear the bowl, we prefer that the wet strength of the tissues of the present invention decays rapidly, exhibiting a normalized cross direction wet tensile of less than about 1/2 the initial value when measured 10 minutes after immersion. To accommodate moistening prior to use, the tissue should retain at least about 15 percent of the initial wet strength value when measured 10 minutes after immersion.
Simple addition of a temporary wet strength agent often produces a paper product that does not possess sufficient softness to be acceptable as a premium bathroom tissue for normal household use. To help bring the softness of the sheet into the premium or near premium range, we have found that it is desirable to vary the jet/wire ratio to make the sheet a little squarer than we normally use in production of wet pressed tissues. For example, in production of conventional wet press tissue, we normally control the jet to wire ratio so that the ratio of machine direction dry tensile strength to cross direction dry tensile strength of the base sheet (before converting and embossing) is about 2.5.
For tissues of the present invention, we prefer to use a jet to wire ratio producing a base sheet having a ratio of MD dry tensile to CD dry tensile of less than about 2.2, more preferably from about 1.6 to 2.1, most preferably from about 1.8 to 1.9. In some instances we may impart slightly more crepe to the web than we would normally use.
Unlike the wet strength agents disclosed in U.S. Ser. No. 08/210,836 filed on Mar. 18, 1994, and U.S. Ser. No. 08/401,690 filed on Mar. 10, 1995, both incorporated herein by reference, the wet strength agents generally do not carry a positive charge and, therefore, cannot be added to the furnish. The wet strength agent can be supplemented by adding a starch to the furnish. To further tailor the properties of the tissue and towel for a particular application cationic softeners/debonders may be added to the furnish or can be added to the web at the same places the wet strength agent is added as shown in FIGS. 2 and 16, at addition points 51, 52, 53, 57, 58, 59, 60, 61, 62, 63, 64 and 65. In some instances, we use the cationic softener/debonder with a temporary wet strength agent. In these circumstances, this mixture can also function as a temporary wet strength agent.